Radiation image storage panel comprising a colorant

ABSTRACT

A radiation image storage panel is provided having a support, an intermediate layer and a phosphor layer comprising a binder and a stimulable phosphor dispersed therein, said panel being colored with a colorant so that the mean reflectance of said panel in the wavelength region of the stimulating rays for said stimulating phosphor is lower than the mean reflectance of said panel in the wavelength region of the light emitted by said stimulable phosphor upon stimulation thereof, characterized in that said colorant is a triarylmethane dye having at least one aqueous alkaline soluble group and is present in at least one of said support, said phosphor layer or an intermediate layer between said support and said phosphor layer.

This application claims the benefit of U.S. Provisional application Ser.No. 60/045,085 filed Apr. 29, 1997.

DESCRIPTION

1. Field of the Invention.

The present invention relates to a radiation image storage panel havinga fluorescent layer comprising a binder and a stimulable phosphordispersed therein.

2. Background of the Invention

In radiography the interior of objects is reproduced by means ofpenetrating radiation which is high energy radiation belonging to theclass of X-rays, γ-rays and high energy elementary particle radiation,e.g. β-rays, electron beam or neutron radiation. For the conversion ofpenetrating radiation into visible light and/or ultraviolet radiationluminescent substances are used called phosphors.

In a conventional radiographic system an X-ray radiograph is obtained byX-rays transmitted imagewise through an object and converted into lightof corresponding intensity in a so-called intensifying screen (X-rayconversion screen) wherein phosphor particles absorb the transmittedX-rays and convert them into visible light and/or ultraviolet radiationwhereto a photographic film is more sensitive than to the direct impactof the X-rays.

In practice the light emitted imagewise by said screen irradiates acontacting photographic silver halide emulsion layer film which afterexposure is developed to form therein a silver image in conformity withthe X-ray image.

As a further development described e.g. in U.S. Pat. No. 3,859,527 anX-ray recording system is disclosed wherein photostimulable storagephosphors are used that in addition to their immediate light emission(prompt emission) on X-ray irradiation, have the property to storetemporarily a large part of the energy of the X-ray image which energyis set free by photostimulation in the form of light different inwavelength characteristic from the light used in the photostimulation.In said X-ray recording system the light emitted on photostimulation isdetected photo-electronically and transformed in sequential electricalsignals.

The basic constituents of such X-ray imaging system operating withstorage phosphors are an imaging sensor containing said phosphor,normally a plate or panel, which temporarily stores the X-ray energypattern, a scanning laser beam for photostimulation, a photo-electroniclight detector providing analog signals that are converted subsequentlyinto digital time-series signals, normally a digital image processorwhich manipulates the image digitally, a signal recorder, e.g. magneticdisk or tape, and an image recorder for modulated light-exposure of aphotographic film or an electronic signal display unit, e.g. cathode raytube. A survey of lasers useful in the read-out of photostimulablelatent fluorescent images is given in the periodical Research DisclosureVolume 308 No. 117 p.991, 1989.

From the preceding description of said two X-ray recording systemsoperating with X-ray conversion phosphor screens in the form of a plateor panel it is clear that said plates or panels serve only asintermediate imaging elements and do not form the final record. Thefinal image is made or reproduced on a separate recording medium ordisplay. The phosphor plates or sheets can be repeatedly re-used. Beforere-use of the photostimulable phosphor panels or sheets a residualenergy pattern is erased by flooding with light.

From the point of view of image quality of the image storage panels,especially with respect to sharpness, the said sharpness does not dependupon the degree of spread of the light emitted by the stimulablephosphor in the panel, but depends on the degree of spread of thestimulable rays in the panel: in order to reduce this spread of light amixture can be made of coarser and finer batches to fill the gapsbetween the coated coarser phosphor particles. A better bulk factor maybe attained by making a mixture of coarser and finer phosphor grainsresulting in a loss in sensitivity unless the said phosphor grains areonly slightly different in sensitivity. For intensifying screens thistopic has already be treated much earlier by Kali-Chemie and has beenpatented in U.S. Pat. Nos. 2,129,295; 2,129,296 and 2,144,040.Radiographs showing improved visualisation, comprising therefore ablue-light absorbing (yellow) dye have been described in EP-A 0 028 521.

Especially the phosphor layer thickness can give rise to increasedunsharpness of the emitted light, this being the more unfavourable ifthe weight ratio between the amount of phosphor particles and the amountof binder decreases for the same coating amount of said phosphorparticles.

Enhancing the weight ratio amount of phosphor to binder to providesharper images, by decreasing the amount of binder leads to unacceptablemanipulation characteristics of the screen due to e.g. insufficientelasticity and brittleness of the coated phosphor layer in the screen.

One way to get thinner coated phosphor layers without changing thecoated amounts of pigment and of binder makes use of a method ofcompressing the coated layer containing both ingredients at atemperature not lower than the softening point or melting point of thethermoplastic elastomer as has been described in EP-A 0 393 662.

Another way free from compression manufacturing techniques has beenproposed in WO 94/0531, wherein the binding medium comprises one or morerubbery and/or elastomeric polymers providing improved elasticity of thescreen, high protection against mechanical damage, high ease ofmanipulation, high pigment to binder ratio and an improved imagequality, especially sharpness.

Early references referring to the improvement of sharpness of radiationimage storage panels are related with the addition of a colorant to thepanels. So in U.S. Pat. No. 4,394,581 a dye or colorant is added to thepanel so that the mean reflectance of said panel in the wavelengthregion of the stimulating rays for said stimulating phosphor is lowerthan the mean reflectance of said panel in the wavelength region of thelight emitted by said stimulable phosphor upon stimulation thereof. InU.S. Pat. No. 4,491,736 more specifically an organic colorant isdisclosed which does not exhibit light emission of longer wavelengththan that of the stimulating rays when exposed thereto. EP-A 0 165 340and the corresponding U.S. Pat. No. 4,675,271 disclose a storagephosphor screen showing a better image definition by incorporation of adye. An analogous effect brought about in phosphor layers of imagestorage panels by incorporation of dyes or colourants has further beendescribed in EP-A 0 253 348 and the corresponding U.S. Pat. No.4,879,202 and in EP-A 0 288 038.

It is however an ever lasting demand to further direct investigations toimprove sharpness.

OBJECTS OF THE INVENTION

Therefore it is an object of the present invention to provide aradiation image storage panel coloured with a dye which gives anexcellent image resolution. Other objects and advantages will becomeclear from the following description and examples.

SUMMARY OF THE INVENTION

In accordance with the present invention a radiation image storage panelis provided having a support, an intermediate layer and a fluorescentlayer comprising a binder and a stimulable phosphor dispersed therein,said panel being colored with a colorant so that the mean reflectance ofsaid panel in the wavelength region of the stimulating rays for saidstimulating phosphor is lower than the mean reflectance of said panel inthe wavelength region of the light emitted by said stimulable phosphorupon stimulation thereof, characterized in that said colorant is atriarylmethane dye having at least one aqueous alkaline soluble groupand is present in at least one of said support, said phosphor layer oran intermediate layer between said support and said phosphor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an absorption signal as a function of wavelength for a dyeof the present invention and a dye of the prior art.

DETAILED DESCRIPTION OF THE INVENTION

In order to fulfill the requirement to prevent scattering of irradiationor rays having a stimulating energy for the storage phosphors coated inthe phosphor layer(s) of the storage panel according to the presentinvention, the choice of a colourant having an absorption as high aspossible in the wavelength range of the stimulating rays and anabsorption as low as possible in the wavelength range of the emittedradiation has lead us to triarylmethane dyes having at least one aqueousalkaline soluble group as perfectly suitable dyes for the purposes setforth above.

From this class of triarylmethane dyes, the most preferred dyes have astructure, the general formula (I) of which is given hereinafter,wherein R¹ -R⁸ each independently represents hydrogen, (substituted orunsubstituted) alkyl or (substituted or unsubstituted) aryl, providedthat at least one of R¹ -R⁸ represents an acidic group like e.g. acarboxylic acid group or a sulphonic acid group in order to providesolubility in alkaline medium in salt form. ##STR1## The formulae II toIV are preferred representatives, given hereinafter ##STR2##

The most preferred dye, corresponding to the formula (II), is providedby Hoechst AG, Germany, in dispersed form and is, due to its performancewith respect to the requirements set forth above, especially suitablefor use in storage panels according to the present invention, oppositeto other blue dyes as e.g. Valifast Blue 1605 and Zapon Fast Blue 3G(manufactured by Hoechst AG), Estrol Brill Blue N-3RL (manufactured bySumitomo Kagaku Co., Ltd.), Sumiacryl Blue F-GSL (manufactured bySumitomo Kagaku Co., Ltd.), D&C Blue No. 1 (manufactured by NationalAnilin Co., Ltd.), Spirit Blue (manufactured by Hodogaya Kagaku Co.,Ltd.), Oil Blue No. 603 (manufactured by Orient Co., Ltd.), Kiton Blue A(manufactured by Ciba-Geigy AG).

As has been shown in FIG. 1 the ratio between absorption signalsmeasured in the wavelength range of the stimulable rays (between 500 and700 nm) and absorption signals in the wavelength range of radiationemitted by the stimulable phosphors upon stimulation (between 350 and450 nm) is clearly in favour of the triarylmethane dyes used in storagepanels according to the present invention if compared with a dye as e.g.the already mentioned Valifast Blue 1605, presented in the said FIG. 1as a comparative dye.

In a preferred embodiment an intermediate light-absorbing layercomprising one or more triarylmethane dye(s) is provided between thephosphor-containing layer and the support and/or in the support itselfin order to avoid scattering of light at the interface of the supportand the layer containing the phosphor layer, thereby increasing theimage resolution of the photostimulable phosphor screen. Particularlysubstituted triarylmethane dyes having a relatively high solubility inprotic or polar solvents as alcohol are preferred as no diffusion to anadjacent phosphor layer, coated from apolar solvents, occurs. Preferredsubstituted triarylmethane dyes therefore are e.g. triphenylmethane dyeswherein at least one phenyl group is substituted with one or moresulphonic acid group, carboxylic group, etc., or wherein, moregenerally, the said colorant is a substituted triphenylmethane dyesoluble aqueous alkaline medium, due to the presence of such anionizable acid group.

When the preferred triarylmethane dye or dyes is(are) present in thesaid light-absorbing intermediate layer the support preferably furthercomprises reflecting particles in order to have reflecting properties. Asupport comprising e.g. TiO₂ (anatase) particles thereby has reflectingproperties, expressed as a degree of reflectance or % reflectance overthe wavelength range from 350 to 600 nm, between 45 and 60%. Otherwise asupport comprising e.g. BaSO₄ particles shows a reflectance percentagebetween 85 and 100%. In another embodiment the said particles areincorporated in a hardened layer coated onto a support. Said hardenenedlayers which should be considered also as intermediate layers betweensupport and phosphor layer preferably comprise the preferred bluetriarylmethane colorant(s) in order to provide a storage panel accordingto the present invention. The presence under the phosphor layer(s) ofthe reflecting layers set forth above, whether or not comprising thepreferred blue colorants, is in favour of screen speed. Although suchreflectance properties could be expected to be disadvantageous withrespect to sharpness, it has now unexpectedly been found that this speedincrease or speed compensation of loss of speed due to the presence ofantihalation dyes is not disadvantageous with respect to imageresolution according to the objects of the present invention.

Another light-reflecting layer which is provided in order to enhance theoutput of light emitted by photostimulation is a (vacuum-deposited)aluminum layer. In terms of reflection according to the presentinvention a dye or colorant should have a mean reflectance in thewavelength region of the stimulating rays for said stimulating phosphorthat is lower than the mean reflectance in the wavelength region of thelight emitted by said stimulable phosphor upon stimulation thereof.

In another embodiment the dye(s) or colorant(s) is(are) additionallypresent in the phosphor layer itself: it is recommended however to addlower amounts of said dyes than in an intermediate layer and/or in thesupport in order to overcome speed decrease.

In still another embodiment the dye(s) or colorant(s) is(are)additionally present in the protective layer coated on top of thephosphor layer: in that case it is recommended to add still loweramounts of said dyes than in the phosphor layer, and correspondinglymuch lower amounts of said dyes in the intermediate layer, in order toprevent further loss in speed of the said screen. Nevertheless itspresence is particularly useful when due to light-piping stimulationlight enters the protecting overcoat layer, causing thereby unsharpness.

In the phosphor layer an increase in the volume ratio of phosphor tobinder further provides a reduction of the thickness of the coatinglayer for an equal phosphor coverage and in addition not only provides abetter sharpness but also offers a higher speed or sensitivity. An extraimprovement in image-sharpness can be realised with the thermoplasticrubber binders cited in WO94/0531 because thinner phosphor layers arepossible at a higher phosphor to binder ratio. Rubbery binders arepreferably chosen because they allow a high volume ratio of pigment tobinder, resulting in excellent physical properties and image quality andin an enhanced speed. In that case a small amount of binding agent doesnot result in too brittle a layer and minimum amounts of binder in thephosphor layer give enough structural coherence to the layer. Especiallyfor storage phosphor members this factor is very important in view ofthe manipulations said member is exposed to. The weight ratio ofphosphor to binder preferably from 80:20 to 99:1. The ratio by volume ofphosphor to binding medium is preferably more than 85/15. In thisconnection a volume ratio of phosphor to binder higher than 92/8 ishardly allowable and is about a maximum value of said volume ratio. Amixture of one or more thermoplastic rubber binders may be used in thecoated phosphor layer(s): preferably the binding medium substantiallyconsists of one or more block copolymers, having a saturated elastomericmidblock and a thermoplastic styrene endblock, as rubbery and/orelastomeric polymers as disclosed in WO 94/00530. Particularly suitablethermoplastic rubbers, used as block-copolymeric binders in phosphorscreens in accordance with the present invention are the KRATON-Grubbers, KRATON being a trade mark name from SHELL. The phosphor layerpreferably has a bound polar functionality of at least 0.5%, a thicknessin the range from 10 to 1000 μm and a ratio by volume of 92:8 or less.

Storage panels as described hereinbefore, according to this invention,may be provided with at least one antioxidant preventing yellowing ofthe screen. The antioxidant(s) is(are) preferably incorporated in thephosphor layer. The coating dispersion may further contain a filler(reflecting or absorbing).

As is well-known the sensitivity of the screen is determined by thechemical composition of the phosphor, its crystal structure and crystalsize properties and the weight amount of phoshor coated in the phosphorlayer. The image quality, particularly sharpness, especially depends onoptical scattering phenomena in the phosphor layer being determinedmainly besides the already mentioned thickness of the phosphor layer bythe packing density. Said packing density of the phosphor particlesdepends on the crystal size distribution of the phosphor particles,their morphology and the amount of binder present in the phosphor layeror layers.

Another factor determining the sensitivity of the screen is thethickness of the phosphor layer, being proportional to the amount ofphosphor(s) coated. Said thickness may be within the range of from 10 to1000 μm, preferably from 50 to 500 μm and more preferably from 100 to300 μm.

The coverage of the phosphor or phosphors present as a sole phosphor oras a mixture of phosphors whether or not differing in chemicalcomposition and present in one or more phosphor layer(s) in a screen ispreferably in the range from about 50 g to 2500 g, more preferably from200 g to 1750 g and still more preferably from 300 to 1500 g/m². Saidone or more phosphor layers may have the same or a different layerthickness and/or a different weight ratio amount of pigment to binderand/or a different phosphor particle size or particle size distribution.It is general knowledge that sharper images with less noise are obtainedwith phosphor particles of smaller mean particle size, but lightemission efficiency declines with decreasing particle size. Thus, theoptimum mean particle size for a given application is a compromisebetween imaging speed and image sharpness desired. Preferred averagegrain sizes of the phosphor particles are in the range of 2 to 30 μm andmore preferably in the range of 2 to 20 μm.

In the phosphor layer(s), any phosphor or phosphor mixture may be coateddepending on the objectives that have to be attained with themanufactured storage phosphor screens. Besides mixing fine grainphosphors with more coarse grain phosphors in order to increase thepacking density, a gradient of crystal sizes may, if required, be buildup in the storage panel. Principally this may be possible by coatingonly one phosphor layer, making use of gravitation forces, but withrespect to reproducibility at least two different storage panels coatedfrom phosphor layers comprising phosphors or phosphor mixtures inaccordance with the present invention may be coated in the presence of asuitable binder, the layer nearest to the support consisting essentiallyof small phosphor particles or mixtures of different batches thereofwith an average grain size of about 5 μm or less and thereover a mixedparticle layer with an average grain size from 8 to 20 μm for thecoarser phoshor particles, the smaller phosphor particles optionallybeing present as interstices of the larger phosphor particles dispersedin a suitable binder. Depending on the needs required the stimulablephosphors in accordance with the present invention or mixtures thereofmay be arranged in a variable way in these coating constructions.

It is clear that within the scope of this invention the choice of thephosphor(s) or phosphor mixture(s) is limited in that the radiationimage storage panel has a wavelength region of the stimulating rayssituated between 500 and 700 nm.

Further in a preferred embodiment according to the present inventionsaid radiation image storage panel has a wavelength region of the lightemitted by said stimulable phosphor upon stimulation thereof situatedbetween 350 and 450 nm.

In radiation image storage panels according to the present inventione.g. divalent europium-doped bariumfluorohalide phosphors may be used,wherein the halide-containing portion may be

(1) stoichiometrically equivalent with the fluorine portion as e.g. inthe phosphor described in U.S. Pat. No. 4,239,968,

(2) may be substoichiometrically present with respect to the fluorineportion as described e.g. in EP-A 0 021 342 or 0 345 904 and U.S. Pat.No. 4,587,036, or

(3) may be superstoichiometrically present with respect to the fluorineportion as described e.g. in U.S. Pat. No. 4,535,237.

According to U.S. Pat. No. 4,239,968 a method is claimed for recordingand reproducing a radiation image comprising the steps of

(i) causing a visible ray- or infrared ray-stimulable phosphor to absorba radiation passing through an object,

(ii) stimulating said phosphor with stimulation rays selected fromvisible rays and infrared rays to release the energy of the radiationstored therein as fluorescent light, characterized in that said phosphoris at least one phosphor selected from the group of alkaline earth metalfluorohalide phosphors.

From the stimulation spectrum of said phosphors it can be learned thatsaid kind of phosphor has high sensitivity to stimulation light of aHe-Ne laser beam (633 nm) but poor photostimulability below 500 nm. Thestimulated light (fluorescent light) is situated in the wavelength rangeof 350 to 450 nm with a peak at about 390 nm (ref. the periodicalRadiology, September. 1983, p.834.).

It can be learned from said U.S. Pat. No. 4,239,968 that it is desirableto use a visible ray (e.g. red light) stimulable phosphor rather than aninfra-red ray-stimulable phosphor because the traps of aninfra-red-stimulable phosphor are shallower than these of the visibleray-stimulable phosphor and, accordingly, the radiation image storagepanel comprising the infra-red ray-stimulable phosphor exhibits arelatively rapid dark-decay (fading).

For solving that problem it is desirable as explained in U.S. Pat. No.4,239,968 to use a photostimulable storage phosphor which has traps asdeep as possible to avoid fading and to use for emptying said trapslight rays having substantially higher photon energy (rays of shortwavelength).

Attempts have been made to formulate phosphor compositions showing astimulation spectrum in which the emission intensity at the stimulationwavelength of 500 nm is higher than the emission intensity at thestimulation wavelength of 600 nm. A suitable phosphor for said purposeis described in U.S. Pat. No. 4,535,238 in the form of a divalenteuropium activated barium fluorobromide phosphor having thebromine-containing portion stoichiometrically in excess of the fluorine.According to U.S. Pat. No. 4,535,238 the photostimulation of thephosphor can proceed effectively with light, even in the wavelengthrange of 400 to 550 nm.

Although BaFBr:Eu²⁺ storage phosphors, used in digital radiography, havea relatively high X-ray absorption in the range from 30-120 keV, whichis a range relevant for general medical radiography, the absorption islower than the X-ray absorption of most prompt-emitting phosphors usedin screen/film radiography, like e.g. LaOBr:Tm, Gd₂ O₂ S:Tb and YTaO₄:Nb. Therefore, said screens comprising light-emitting luminescentphosphors will absorb a larger fraction of the irradiated X-ray quantathan BaFBr:Eu screens of equal thickness. The signal to noise ratio(SNR) of an X-ray image being proportional to the square-root of theabsorbed X-ray dose, the images made with the said light-emittingscreens will consequently be less noisy than images made with BaFBr:Euscreens having the same thickness. A larger fraction of X-ray quantawill be absorbed when thicker BaFBr:Eu screens are used. Use of thickerscreens, however, leads to diffusion of light over larger distances inthe screen, which causes deterioration of image resolution. For thisreason, X-ray images made with digital radiography, using BaFBr screens,as disclosed in U.S. Pat. No. 4,239,968, give a more noisy impressionthan images made with screen/film radiography.

A more appropriate way to increase the X-ray absorption of phosphorscreens is by increasing the intrinsic absorption of the phosphor. InBaFBr:Eu storage phosphors this can be achieved by partly substitutingbromine by iodine.

BaFX:Eu phosphors containing large amounts of iodine have been describede.g. in EP-A 0 142 734, the general formula of said phosphor isBaF(Br_(1-x) I_(x)):yEu, and 10⁻³ ≦x<1.0. In FIG. 3 of said patentapplication, the relative luminance of BaFX:Eu storage phosphors isshown as a function of the iodine content. It is clear from said FIG. 3of the disclosure mentioned above, that, although x can be as great as1.0 according to the general formula, the portion of Br that is replacedby I should preferably not be made higher than 50% since replacement ofa larger portion of Br by I, leads to a lower relative luminance, of thelight emitted upon stimulation. The relative luminance of the storagephosphor should be as high as possible, since the sensitivity of astorage phosphor system is proportional to the storage phosphorluminance and apart from a high X-ray absorption, a high systemsensitivity is essential for reducing image noise. Therefore, in aphosphor as disclosed in EP-A 0 142 734, the gain in image quality, dueto the higher absorption of X-rays when more than 50% of iodine isincluded in the phosphor is offset by the lowering of the relativeluminance.

Divalent europium activated barium fluorobromide phosphors suitable foruse according to the present invention have further been described inEP-A 0 533 236 and in the corresponding U.S. Pat. Nos. 5,422,220 and5,547,807. In the said EP-A 0 533 236 a divalent europium activatedstimulable phosphor is claimed wherein the stimulated light has a higherintensity when the stimulation proceeds with light of 550 nm, than whenthe stimulation proceeds with light of 600 nm. It is said that in saidphosphor a "minor part" of bromine is replaced by chlorine and/oriodine. By minor part has to be understood less than 50 atom %.

Still other divalent europium activated barium fluorobromide phosphorssuitable for use according to the present invention have been describedin EP-A 0 533 234. In this EP-A 0 533 234 a process is described toprepare europium-doped alkaline earth metal fluorobromide phosphors,wherein fluorine is present in a larger atom % than bromine, and whichhave a stimulation spectrum that is clearly shifted to the shorterwavelength region. Therein use of shorter wavelength light in thephotostimulation of phosphor panels containing phosphor particlesdispersed in a binder is in favour of image-sharpness since thediffraction of stimulation light in the phosphor-binder layer containingdispersed phosphor particles acting as a kind of grating will decreasewith decreasing wavelength.

As is apparent from the examples in this EP-A 0 533 234 the ultimatelyobtained phosphor composition determines the optimum wavelength for itsphotostimulation and, therefore, the sensitivity of the phosphor in aspecific scanning system containing a scanning light source emittinglight in a narrow wavelength region.

Other preferred photostimulable phosphors according to the applicationsmentioned hereinbefore contain an alkaline earth metal selected from thegroup consisting of Sr, Mg and Ca with respect to barium in an atompercent in the range of 10⁻¹ to 20 at %. From said alkaline earth metalsSr is most preferred for increasing the X-ray conversion efficiency ofthe phosphor. Therefore in a preferred embodiment strontium isrecommended to be present in combination with barium and fluorinestoichiometrically in larger atom % than bromine alone or brominecombined with chlorine and/or iodine.

Other preferred photostimulable phosphors according to that applicationcontain a rare earth metal selected from the group consisting of Ce, Pr,Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu with respect to barium in an atompercent in the range of 10⁻³ to 10⁻¹ at %. From said rare earth metalsGd is preferred for obtaining a shift of the maximum of thephotostimulation spectrum of the phosphor to the shorter wavelengths.

The preferred phosphors of that application referred to hereinbefore arealso preferred for use in the present invention the proviso that, as setforth hereinbefore, the wavelength region of the stimulating rays isbetween 500 and 700 nm.

Still other preferred photostimulable phosphors for use according to thepresent invention contain a trivalent metal selected from the groupconsisting of Al, Ga, In, Tl, Sb, Bi and Y with respect to barium in anatom percent in the range of 10⁻¹ to 10 at %. From said trivalent metalsBi is preferred for obtaining a shift of the maximum of thephotostimulation spectrum of the phosphor to the shorter wavelengths.

Preferred phosphors for use according to this invention are furtherphosphors wherein fluorine is present stoichiometrically in a largeratom % than bromine taken alone or bromine combined with chlorine and/oriodine, e.g. fluorine is present in 3 to 12 atom % in excess overbromine or bromine combined with chlorine and/or iodine.

Still other particularly suitable barium fluorobromide phosphors for useaccording to the present invention contain in addition to the maindopant Eu²⁺ at least Sm as codopant as described in EP-A 0 533 233 andin the corresponding U.S. Pat. No. 5,629,125.

Still other useful phosphors are those wherein Ba-ions are partiallyreplaced by Ca-ions at the surface of the phosphors have been describedin EP-A 0 736 586.

In digital radiography it can be advantageous to use photostimulablephosphors that can very effectively be stimulated by light withwavelength higher than 600 nm as for phosphors included for use instorage panels according to the present invention, since then the choiceof small reliable lasers that can be used for stimulation (e.g. He-Ne,semi-conductor lasers, solid state lasers, etc) is very great so thatthe laser type does not dictate the dimensions of the apparatus forreading (stimulating) the stimulable phosphor screen.

More recently stimulable phosphors, giving a better signal-to-noiseratio, a higher speed, further being stimulable at wavelengths above 600nm have therefore been described in EP-Application No. 96202816, filedOct. 10, 1996. Therein a storage phosphor class has been describedproviding high X-ray absorption, combined with a high intensity ofphotostimulated emission, thus allowing to build a storage phosphorsystem for radiography yielding images that have at the same time a highsharpness and a low noise content, through a decreased level of X-rayquantum noise and a decreased level of fluorescence noise. Further saidclass of photostimulable phosphors provides a high X-ray absorption,combined with a high intensity of photostimulated emission, showing saidhigh intensity of photostimulated emission when stimulated with lighthaving a wavelength above 600 nm. Said photostimulable phosphors canfurther be used in panels for medical diagnosis, whereby the dose ofX-ray administered to the patient can be lowered and the image qualityof the diagnostic image enhanced: in a panel including said phosphor indispersed form on photostimulation with light in the wavelength rangeabove 600 nm images with very high signal-to-noise ratio are yielded.

Said photostimulable phosphors are within the scope of the followingformula:

    Ba.sub.1-x-y-p-3q-z Sr.sub.x M.sub.y.sup.2+ M.sub.2p.sup.1+ M.sub.2q.sup.3+ F.sub.2-a-b Br.sub.a I.sub.b :zEu

wherein:

M¹⁺ is at least one alkali metal selected from the group consisting ofLi, Na, K, Rb and Cs;

M²⁺ is at least one divalent metal selected from the group consisting ofCa Mg and Pb;

M³⁺ is at least one trivalent metal selected from the group consistingof Al, Ga, In, Tl, Sb, Bi, Y or a trivalent lanthanide,

e.g. La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;

0≦x≦0.30, 0≦y≦0.10, 10⁻⁶ ≦z≦0.2, 0≦p≦0.3, 0≦q≦0.1, 0.05≦a≦0.76,0.20≦b≦0.90 and a+b<1.00.

In a preferred embodiment 0.06≦x≦0.20 and 0.85≦a+b≦0.96 and in a furtherpreferred embodiment, 0.06≦x≦0.20, 0.85≦a+b≦0.96, M¹⁺ is Cs or Rb, M²⁺is Pb, 10⁻⁴ ≦y≦10⁻³, 10⁻⁴ ≦p≦10⁻¹ and q=0.

A very useful and preferred method for the preparation of stimulablephosphors can be found in Research Disclosure Volume 358, February 1994p 93 item 35841, which is incorporated herein by reference.

In order to produce phosphors with a constant composition and,therefore, with a constant stimulation spectrum for use in storagephosphor panels, even in the presence of co-dopants that influence theposition of the stimulation spectrum as e.g. samarium or an alkalimetal, added to the raw mix of base materials in small amounts asprescribed in EP-A 0 533 234, a solution therefore has been proposed inU.S. Pat. No. 5,517,034.

Therein a method of recording and reproducing a penetrating radiationimage has been proposed comprising the steps of:

(i) causing stimulable storage phosphors to absorb said penetratingradiation having passed through an object or emitted by an object and tostore energy of said penetrating radiation,

(ii) stimulating said phosphors with stimulating light to release atleast a part of said stored energy as fluorescent light and

(iii) detecting said stimulation light, characterized in that saidphosphors consist of a mixture of two or more individually prepareddivalent europium doped bariumfluorohalide phosphors at least one ofwhich contains (a) co-dopant(s) which co-determine(s) the character ofthe stimulation spectrum of the co-doped phosphor.

Further particularly suitable divalent europium barium fluorobromidephosphors for use according to that invention correspond to theempirical formula (I) of EP-A 0 533 236 and contain in addition to themain dopant Eu²⁺ at least one alkali metal, preferably sodium orrubidium, as a co-dopant. Preferred photostimulable phosphors accordingto that application contain samarium with respect to barium in an atompercent in the range of 10⁻³ to 10 at %. Other preferred photostimulablephosphors according to that application contain an alkali metal selectedfrom the group consisting of Li, Na, K, Rb and Cs, with respect tobarium in an atom percent in the range of 10⁻² to 1 at %.

In praxis a maximum in the stimulation spectrum for e.g. lithium fluxedstimulable europium activated bariumfluorohalide phosphor can be foundbetween 520 and 550 nm, whereas for cesium fluxed phosphor its maximumis situated between 570 and 630 nm. Maxima for the stimulation spectraof said phosphors after making a mixture thereof can be found atintermediate wavelengths. The stimulation spectrum of said mixture isfurther characterized in that the emission intensity at 500 nmstimulation is always lower than the emission intensity at 600 nm. Thebroadening of the obtained stimulation spectra is a further advantageresulting from the procedure of making blends in that the storage panelin which the stimulable phosphors are incorporated is sensitive to abroad region of stimulation wavelengths in the visible range of thewavelength spectrum. As a consequence the storage panel comprising alayer with the phosphor blends described hereinbefore may offeruniversal application possibilities from the point of view ofstimulation with different stimulating light sources. Differentstimulating light sources that may be applied are those that have beendescribed in Research Dislosure No. 308117, December 1989.

A radiographic screen according to the present invention can be preparedby the following manufacturing process.

The phosphor layer can be applied to the support by any coatingprocedure, making use of solvents for the binder of the phosphorcontaining layer as well as of useful dispersing agents, usefulplasticizers, useful fillers and subbing or interlayer layercompositions that have been described in extenso in the EP-A 0 510 753.

Phosphor particles may be mixed with dissolved rubbery and/orelastomeric polymers, in a suitable mixing ratio in order to prepare adispersion. Said dispersion is uniformly applied to a substrate by aknown coating technique as e.g. doctor blade coating, roll coating,gravure coating or wire bar coating, and dried to form a luminescentlayer fluorescing by X-ray irradiation and called hereinafterfluorescent layer. Further mechanical treatments like compression tolower the void ratio is not required within the scope of the presentinvention.

Useful dispersing agents to improve the dispersibility of the phosphorparticles dispersed into the coating dispersion are described in EP-A 0510 753 as well as a variety of additives that can be added to thephosphor layers such as a plasticizer for increasing the bonding betweenthe binder and the phosphor particles in the phosphor layer and,according to the present invention, to a light-reflecting or absorbingfiller and/or a colourant.

Useful plasticizers include phosphates such as triphenyl phosphate,tricresyl phosphate and diphenyl phosphate; phthalates such as diethylphthalate and dimethoxyethyl phthalate; glycolates such as ethylphthalylethyl glycolate and butylphthalyl butyl glycolate; polymeric plastizers,e.g. and polyesters of polyethylene glycols with aliphatic dicarboxylicacids such as polyester of triethylene glycol with adipic acid andpolyester of diethylene glycol with succinic acid.

The stimulable phosphor is preferably protected against the influence ofmoisture by adhering thereto chemically or physically a hydrophobic orhydrophobizing substance. Suitable substances for said purpose aredescribed e.g. in U.S. Pat. No. 4,138,361.

In the composition of a storage panel, one or more additional layers areoccasionally provided between the support and the phosphor containinglayer, having subbing or interlayer layer compositions, in order toimprove the bonding between the support and the phosphor layer, or inorder to improve the sensitivity of the screen or the sharpness andresolution of an image provided thereby. For instance, a subbing layeror an adhesive layer may be provided by coating polymer material overthe surface of the support on the phosphor layer side.

Additional layer(s) may be coated on the support either as a backinglayer or interposed between the support and the intermediate layer, thesaid intermediate layer and the phosphor containing layer(s). Several ofsaid additional layers may be applied in combination.

In the preparation of the phosphor screen having a primer layer betweenthe substrate and the layer containing the phosphor(s), the primer layeris provided on the substrate beforehand, and then the phosphordispersion is applied to the primer layer and dried to form thefluorescent layer.

When the phosphors are used in combination with a binder to prepare ascreen or a panel according to the present invention, the phosphorparticles are intimately dispersed in a solution of the binder and thencoated on the support and dried. The coating of the present phosphorbinder layer may proceed according to any usual technique, e.g. byspraying, dip-coating or doctor blade coating. After coating, thesolvent(s) of the coating mixture is (are) removed by evaporation, e.g.by drying in a hot (60° C.) air current.

An ultrasonic treatment can be applied to improve the packing densityand to perform the de-aeration of the phosphor-binder combination.Before the optional application of a protective coating thephosphor-binder layer may be calendered to improve the packing density(i.e. the number of grams of phosphor per cm³ of dry coating).

After applying the coating dispersion onto the support, the coatingdispersion is heated slowly to dryness in order to complete theformation of a phosphor layer. In order to remove as much as possibleentrapped air in the phosphor coating composition it can be subjected toan ultra-sonic treatment before coating.

After the formation of the phosphor layer, a protective layer isgenerally provided on top of the fluorescent layer.

Correlating features of roughness and thickness of the protectivecoating conferring to the screens of the present invention havingdesirable and unexpected properties of ease of manipulation andexcellent image sharpness have been described in the EP-A 0 510 754.

According to a preferred embodiment the coating of the protective layerhere proceeds by screen-printing (silk-screen printing).

The protective coating composition may be applied by a rotary screenprinting device as has been described in detail in the said EP-A 0 510753. Very useful radiation curable compositions for forming a protectivecoating contain as primary components:

(1) a crosslinkable prepolymer or oligomer,

(2) a reactive diluent monomer, and in the case of an UV curableformulation

(3) a photoinitiator.

Examples of suitable prepolymers for use in a radiation-curablecomposition applied to the storage panel according to the presentinvention are the following: unsaturated polyesters, e.g. polyesteracrylates; urethane modified unsaturated polyesters, e.g.urethane-polyester acrylates. Liquid polyesters having an acrylic groupas a terminal group, e.g. saturated copolyesters which have beenprovided with acryltype end groups are described in published EP-A 207257 and Radiat. Phys. Chem., Vol. 33, No. 5, 443-450 (1989). The latterliquid copolyesters are substantially free from low molecular weight,unsaturated monomers and other volatile substances and are of very lowtoxicity (ref. the journal Adhasion 1990 Heft 12, page 12). Thepreparation of a large variety of radiation-curable acrylic polyestersis given in German Offenlegungsschrift No. 2838691. Mixtures of two ormore of said prepolymers may be used. A survey of UV-curable coatingcompositions is given e.g. in the journal "Coating" 9/88, p. 348-353.

When the radiation-curing is carried out with ultraviolet radiation(UV), a photoinitiator is present in the coating composition to serve asa catalyst to initiate the polymerization of the monomers and theiroptional cross-linking with the pre-polymers resulting in curing of thecoated protective layer composition.

A photosensitizer for accelerating the effect of the photoinitiator maybe present. Photoinitiators suitable for use in UV-curable coatingcompositions belong to the class of organic carbonyl compounds, forexample, benzoin ether series compounds such as benzoin isopropyl,isobutylether; benzil ketal series compounds; ketoxime esters;benzophenone series compounds such as benzophenone,o-benzoylmethylbenzoate; acetophenone series compounds such asacetophenone, trichloroacetophenone, 1,1-dichloroacetophenone,2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone;thioxanthone series compounds such as 2-chlorothioxanthone,2-ethylthioxanthone; and compounds such as2-hydroxy-2-methylpropiophenone,2-hydroxy-4'-isopropyl-2-methylpropiophenone,1-hydroxycyclohexylphenylketone; etc.

A particularly preferred photoinitiator is2-hydroxy-2methyl-1-phenyl-propan-1-one which product is marketed by E.Merck, Darmstadt, Germany, under the tradename DAROCUR 1173. The abovementioned photopolymerisation initiators may be used alone or as amixture of two or more. Examples of suitable photosensitisers areparticular aromatic amino compounds as described e.g. in GB-A 1,314,556,1,486,911, U.S. Pat. No. 4,255,513 and merocyanine and carbostyrilcompounds as described in U.S. Pat. No. 4,282,309.

When using ultraviolet radiation as curing source the photoinitiatorwhich should be added to the coating solution will to a more or lessextent also absorb the light emitted by the phosphor thereby impairingthe sensitivity of the radiographic screen, particularly when a phosphoremitting UV or blue light is used. Electron beam curing may therefore bemore effective.

The protective coating of the present storage panel is given an embossedstructure following the coating stage by passing the uncured or slightlycured coating through the nip of pressure rollers wherein the rollercontacting said coating has a micro-relief structure, e.g. giving thecoating an embossed structure so as to obtain relief parts as has beendescribed e.g. in EP-A's 455 309 and 456 318.

A suitable process for forming a textured structure in a plastic coatingby means of engraved chill roll is described in U.S. Pat. No. 3,959,546.

According to another embodiment the textured or embossed structure isobtained already in the coating stage by applying the paste-like coatingcomposition with a gravure roller or screen printing device operatingwith a radiation-curable liquid coating composition theHoeppler-viscosity of which at a coating temperature of 25° C. isbetween 450 and 20,000 mpa.s.

To avoid flattening of the embossed structure under the influence ofgravitation, viscosity and surface shear the radiation-curing iseffected immediately or almost immediately after the application of theliquid coating. The rheologic behaviour or flow characteristics of theradiation-curable coating composition can be controlled by means ofso-called flowing agents. For that purpose alkylacrylate estercopolymers containing lower alkyl (C1-C2) and higher alkyl (C6-C18)ester groups can be used as shear controlling agents lowering theviscosity. The addition of pigments such as colloidal silica raises theviscosity.

A variety of other optional compounds can be included in theradiation-curable coating composition of the present radiographicarticle such as compounds to reduce static electrical chargeaccumulation, plasticizers, matting agents, lubricants, defoamers andthe like as has been described in EP-A 0 510 753. In that document adescription has also been given of the apparatus and methods for curing,as well as a non-limitative survey of X-ray conversion screen phosphors,of photostimulable phosphors and of binders of the phosphor containinglayer.

The edges of the screen, being especially vulnerable by multiplemanipulation, may be reinforced by covering the edges (side surfaces)with a polymer material being formed essentially from amoisture-hardened polymer composition prepared according to EP-A 0 541146.

Support materials for radiographic screens which in accordance withspecific embodiments of the present invention are preferably plasticfilms such as films of cellulose acetate, polyvinyl chloride, polyvinylacetate, polyacrylonitrile, polystyrene, polyester, polyethyleneterephthalate, polyethylene naphthalate, polyamide, polyimide, cellulosetriacetate and polycarbonate; metal sheets such as aluminum foil andaluminum alloy foil; ordinary papers; baryta paper; resin-coated papers;pigment papers containing titanium dioxide or the like; and papers sizedwith polyvinyl alcohol or the like.

Examples of preferred supports include polyethylene terephthalate, clearor blue coloured or black coloured (e.g., LUMIRROR C, type X30 suppliedby Toray Industries, Tokyo, Japan), polyethylene terephthalate filledwith TiO₂ or with BaSO₄. Metals as e.g. aluminum, bismuth and the likemay be deposited e.g. by vaporization techniques to get a polyestersupport having radiation-reflective properties.

These supports may have thicknesses which may differ depending on thematerial of the support, and may generally be between 50 and 1000 μm,more preferably between 80 and 500 μm depending on handling properties.Further are mentioned glass supports and metal supports.

Normally the screens described hereinbefore are applied for medicalX-ray diagnostic applications but according to a particular embodimentthe present radiographic screens may be used in non-destructive testing(NDT), of metal objects, where more energetic X-rays and γ-rays are usedthan in medical X-ray applications. Especially in the said applicationsfurther glass and metal supports are used, the latter preferably of highatomic weight, as described e.g. in U.S. Pat. Nos. 3,872,309 and3,389,255.

According to a particular embodiment for industrial radiography theimage-sharpness of the phosphor screen is improved by incorporating inthe phosphor screen between the phosphor-containing layer and thesupport and/or at the rear side of the support a pigment-binder layercontaining a non-fluorescent pigment being a metal compound, e.g. saltor oxide of lead, as described in Research Disclosure September 1979,item 18502.

In order to obtain a reasonable signal-to-noise ratio (S/N) thestimulation light should be prevented from being detected together withthe fluorescent light emitted on photostimulation of the storagephosphor. Therefore a suitable filter means is used preventing thestimulation light from entering the detecting means, e.g. aphotomultiplier tube. Because the intensity ratio of the stimulationlight is markedly higher than that of the stimulated emission light,i.e. differing in intensity in the range of 10⁴ :1 to 10⁶ :1 (seepublished EP-A 0 007 105, column 5) a very selective filter should beused. Suitable filter means or combinations of filters may be selectedfrom the group of : cut-off filters, transmission bandpass filters andband-reject filters. A survey of filter types and spectral transmittanceclassification is given in SPSE Handbook of Photographic Science andEngineering, Edited by Woodlief Thomas, Jr.--A Wiley-IntersciencePublication--John Wiley & Sons, New York (1973), p. 264-326.

The fluorescent light emitted by photostimulation is detected preferablyphoto-electronically with a transducer transforming light energy intoelectrical energy, e.g. a phototube (photomultiplier) providingsequential electrical signals that can be digitized and stored. Afterstorage these signals can be subjected to digital processing. Digitalprocessing includes e.g. image contrast enhancement, spatial frequencyenhancement, image subtraction, image addition and contour definition ofparticular image parts.

According to one embodiment for the reproduction of the recorded X-rayimage the optionally processed digital signals are transformed intoanalog signals that are used to modulate a writing laser beam, e.g. bymeans of an acousto-optical modulator. The modulated laser beam is thenused to scan a photographic material, e.g. silver halide emulsion filmwhereon the X-ray image optionally in image-processed state isreproduced.

According to another embodiment the digital signals obtained from theanalog-digital conversion of the electrical signals corresponding withthe light obtained through photostimulation are displayed on acathode-ray tube. Before display the signals may be processed bycomputer. Conventional image processing techniques can be applied toreduce the signal-to-noise ratio of the image and enhance the imagequality of coarse or fine image features of the radiograph.

The invention is illustrated by the following examples without howeverlimiting it thereto. Important concerning image quality as reflected inS-SWR measuring methods will be described hereinafter in the examples.

EXAMPLES

Definitions and methods used.

Measurement of sensitivity S and square wave response SWR for stimulablephosphor screens

For the photostimulable phosphor screens coated with BaSrFBr:Eu²⁺phosphor the measurement of S and SWR was carried out with an imagescanner made up with a He-Ne laser. The beam of a 10 mW red He-Ne laseris focussed to a small spot of 140 μm (FMWH) with an optic containing abeamexpander and a collimating lens. A mirror galvanometer is used toscan this small laserspot over the entire width of a phosphor sample.During this scanning procedure the phosphor is stimulated and theemission light is captured by an array of optical fibers which are sitedon one line. At the other end of the optical fibers being mounted in acircle a photomultiplier is situated.

To attenuate the stimulating light an optical filter, type BG3 fromSCHOTT, is placed between the fiber and the photomultiplier. In this wayonly the light emitted by the phosphor is measured. The small current ofthe photomultiplier is first amplified with an I/V convertor anddigitalised with an A/D convertor.

The measuring set up is connected with a HP 9826 computer and a HP 6944multiprogrammer to controll the measurement. Starting the procedure anelectronic shutter is closed to shut down the laser. A phosphor samplemeasuring 50 mm×210 mm is excited with a 85 kV X-ray source providedwith an aluminum filter having a thickness of 21 mm. The radiation doseis measured with a FARMER dosemeter. Between the X-ray source and thephosphor layer a thin lead-raster containing 6 different spatialfrequencies is mounted to modulate the X-ray radiation. Frequencies usedare 0.50, 1.00, 2.00 and 3.00 line pairs per mm. After exposure thesample is put into the laser scanner. To read out one line the shutteris opened and the galvanometer is moved linearly. During the scanningprocedure the emitted light is measured continuously with the A/Dconvertor at a sampling rate frequency of 100 kHz and stored within amemory card in the multiprogrammer. One scan thus contains 100000pixels. Once the scan is complete the shutter is closed again and thegalvanometer is put on his original position again.

The data of the scan line are transferred from the memory card in themultiprogrammer to the computer where said data are analysed. A firstcorrection takes into account the sensitivity variation of the scan linewith the distance. Therefore a calibration scan was measured previouslyfor a phosphor sample that was exposed quite homogeneously. A secondcorrection takes into account the amount of X-ray dose by dividing saidvalues by the said dose amount.

The different blocks are separated and the amplitude on each spatialfrequency is calculated, making use of Fourier analysis. The amplitudeof the first block having a spatial frequency of 0.025 line pairs per mmis taken as the sensitivity of the stimulable phosphor screen. The othervalues are the results for the curve of the Square Wave Response (SWR:SWRl referring to the response at 1 line pair per mm; SWR2 to theresponse at 2 line pairs per mm) which is representative for theresolution of the screen.

Composition of the screens

    ______________________________________    Antihalation undercoat layer:    Solution A:    MOWILITH CT5 (from HOECHST AG)                             300         g    Ethanol                  700         g    CYMEL 300                60          g    p-toluene sulphonic acid 12          g    Solution B:    Dye-1                    0.750       g    Ethanol                  150         g    Sodium hydroxide         0.08        g    *16 hours after its preparation solution B is filtered off:    a red-brown solution is obtained.    Coating solution:    Solution A               33.3        g    Solution B               3.0         g    Ethanol                  63.6        g    ______________________________________

The coating solution was coated by dipcoating techniques at a rate of 4m per minute on a polyethylene terephthalate support (see furtherTable 1) having reflecting properties (containing BaSO₄ particles) orabsorbing properties (having carbon black particles). Thermal curing wasperformed over one night at 80° C. after drying.

Properties of the thus obtained antihalation layer.

An absorption of 0.22 at a wavelength of 633 nm (HeNe laser emissionwavelength). No substantial absorption is measured at the emissionwavelength of the stimulable phosphor (having its maximum emission at390 nm).

    ______________________________________    Phosphor layer composition:    BAEROSTAB M36 (from Barlocher GmbH)                               1.5         g    DISPERSE AYD 9100 (from Daniel Produkts Company)                               0.75        g    KRATON FG19101X (from Shell Chemicals)                               12.5        g    BaSrFBr:Eu (mean particle size: 7 μm)                               270         g    BaSrFBr:Eu (mean particle size: 3 μm)                               30          g    ______________________________________

Preparation of the phosphor laquer composition:

BAEROSTAB M36, DISPERSE AYD 9100 and KRATON FG19101X were dissolvedwhile stirring in the prescribed amounts in 61.5 g of a solvent mixturefrom wash benzine 100-120, toluene and butyl acrylate in ratios byvolume of 50:30:20. The phosphors were added thereafter and stirring wasfurther proceeded for another 10 minutes at a rate of 1700 r.p.m..

The composition was doctor blade coated at a coating rate of 2.5 m perminute onto a subbed 175 μm thick polyethylene terephthalate support anddried at room temperature during 30 minutes. In order to remove volatilesolvents as much as possible the coated phosphor plate was dried at 90°C. in a drying stove.

It has been established that a layer composition was obtained havinggood curable properties. Moreover no diffusion of colorant was foundfrom the intermediate antihalation layer between support and phosphorlayer to the phosphor layer: this particular dye is advantageouslysoluble in ethanol but insoluble in solvents present in the coatingcomposition of the phosphor layer.

In Table I the coating composition is given for the stimulable phosphor.For each screen sample the following data referring to the compositionare summarised in the respective Tables:

number of the sample (Matl. No.)

support: expressed as % reflectance: 0-10% corresponds to a supporthaving carbon black dispersed in the support material; 85-100% to asupport having BaSO⁴ dispersed therein;

presence of antihalation undercoat layer (AHU): yes (Y) or no (N);

phosphor coverage weight (PCW) (between 70 and 90 mg/cm²).

presence (N=No) and, if present, amount of dye in the phosphor layer;

screen speed (a higher figure refers to a more sensitive screen);

SWR1 and SWR2 values respectively.

                                      TABLE I    __________________________________________________________________________    Stimulable phosphor screens with BaSrFBr:Eu.sup.2+  as a    stimulable phosphor.    Matl.        Support   PCW    No. (% refl.)              AHU (mg/cm.sup.2)                       DYE   SPEED                                 SWR1 SWR2    __________________________________________________________________________    1   85-100              Y   69   N     135 .70  .37    2   85-100              Y   78   N     150 .67  .34    3   0-10  N   79   N     132 .68  .36    4   0-10  N   90   N     146 .66  .33    5   0-10  N   85   N     143 .67  .34    6   0-10  N   86   0.006%*                             138 .68  .36    7   0-10  N   87   0.06%*                             108 .73  .43    __________________________________________________________________________     *solution B (in volume %)

As is clear from the data related with speed and sharpness the highest(most preferred) values are attained for materials Nos. 1 and 2 (bothhaving an antihalation undercoat and a reflective support): an increasedamount of phosphor coated makes speed increase, without a remarkabledecrease in sharpness. Addition of antihalation dye in minor amounts tothe phosphor layer alone as in material No. 6 having no reflection layeralso leads to a suitably good relationship between speed and sharpness.

I claim:
 1. A radiation image storage panel having a support, anintermediate layer and a phosphor layer comprising a binder and astimulable phosphor dispersed therein, said panel being colored with acolorant so that the mean reflectance of said panel in the wavelengthregion of the stimulating rays for said stimulating phosphor is lowerthan the mean reflectance of said panel in the wavelength region of thelight emitted by said stimulable phosphor upon stimulation thereof,characterized in that said colorant is a triarylmethane dye having atleast one aqueous alkaline soluble group and is present in at least oneof said support, said phosphor layer or an intermediate layer betweensaid support and said phosphor layer.
 2. A radiation image storage panelaccording to claim 1, wherein the said colorant is a triphenylmethanedye according to the general formula (I), ##STR3## wherein R¹ -R⁸ eachindependently represents hydrogen, alkyl or aryl, provided that at leastone of R¹ -R⁸ represents an acidic group.
 3. A radiation image storagepanel according to claim 1, wherein the said colorant has a structurecorresponding to formula (II) ##STR4##
 4. A radiation image storagepanel according to claim 1, wherein the said support has a reflectancepercentage of from 45 to 60% in the wavelength range from 350 to 600 nm.5. A radiation image storage panel according to claim 1, wherein thesaid intermediate layer has a reflectance percentage of from 85 to 100%in the wavelength range from 350 to 600 nm.
 6. A radiation image storagepanel according to claim 1, wherein the said support is covered with analuminum layer which is present between the said support and the saidintermediate layer.
 7. A radiation image storage panel according toclaim 1, wherein the wavelength region of the stimulating rays isbetween 500 and 700 nm .
 8. A radiation image storage panel according toclaim 1, wherein the wavelength region of light emitted by saidstimulable phosphor upon stimulation thereof is between 350 and 450 nm.9. A radiation image storage panel according to claim 1, wherein thesaid binder is a rubbery binder substantially consisting of one or moreblock copolymers, having a saturated elastomeric midblock and athermoplastic styrene endblock, as rubbery and/or elastomeric polymers.10. A radiation image storage panel according to claim 1, wherein saidstimulable phosphor has a composition according to the formula

    Ba.sub.1-x-y-p-3q-z Sr.sub.x M.sub.y.sup.2+ M.sub.2p.sup.1+ M.sub.2q.sup.3+ F.sub.2-a-b Br.sub.a I.sub.b :zEu

wherein: M¹⁺ is at least one alkali metal selected from the groupconsisting of Li, Na, K, Rb and Cs; M²⁺ is at least one divalent metalselected from the group consisting of Ca, Mg and Pb; M³⁺ is at least onetrivalent metal selected from the group consisting of Al, Ga, In, Tl,Sb, Bi, Y and a trivalent lanthanide; 0≦x≦0.30, 0≦y≦0.10, 10⁻⁶ ≦z≦0.2,0≦p≦0.3, 0≦q≦0.1, 0.05≦a≦0.76, 0.20≦b≦0.90 and a+b<1.00.
 11. A radiationimage storage panel according to claim 10, wherein said trivalentlanthanide is selected from the group consisting of La, Ce, Pr, Nd, Sm,Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
 12. A radiation image storage panelaccording to claim 1, wherein said stimulable phosphor has a compositionaccording to the formula

    Ba.sub.1-x-y-p-3q-z Sr.sub.x M.sub.y.sup.2+ M.sub.2p.sup.1+ M.sub.2q.sup.3+ F.sub.2-a-b Br.sub.a I.sub.b :zEu

wherein: M¹⁺ is at least one alkali metal selected from the groupconsisting of Li, Na, K, Rb and Cs; M²⁺ is at least one divalent metalselected from the group consisting of Ca, Mg and Pb; M³⁺ is at least onetrivalent metal selected from the group consisting of Al, Ga, In, Ti,Sb, Bi, Y and a trivalent lanthanide; 0.06≦x≦0.20, 0≦y≦0.10, 10⁻⁶≦z≦0.2, ≦ p≦0.3, 0≦q≦0.1, 0.05≦a≦0.76, 0.20≦b≦0.90 and 0.85≦a+b≦0.96.13. A radiation image storage panel according to claim 1, wherein saidstimulable phosphor has a composition according to the formula

    Ba.sub.1-x-y-p-3q-z Sr.sub.x M.sub.y.sup.2+ M.sub.2p.sup.1+ M.sub.2q.sup.3+ F.sub.2-a-b Br.sub.a I.sub.b :zEu

wherein: M¹⁺ is Rb or Cs; M²⁺ is Pb; M³⁺ is at least one trivalent metalselected from the group consisting of Al, Ga, In, Tl, Sb, Bi, Y and atrivalent lanthanide; 0.06≦x≦0.20, 10⁻⁴ ≦y≦10⁻³, 10⁻⁴ ≦p≦10⁻¹, q=0, 10⁻⁶≦z≦0.2, 0.05≦a≦0.76, 0.20≦b≦0.90, and 0.85≦a+b≦0.96.